Many synthetic organic polymers are degraded by exposure to light resulting in discoloration and/or embrittlement. It is known that actinic radiation, particularly in the near ultraviolet region, has a deleterious effect on the appearance and properties of organic polymers. For example, normally colorless or light colored polyesters and cellulostics such as cellulose acetate yellow on exposure to sun light. Polystyrene discolors and cracks with an accompanying loss of the desirable physical properties when exposed to actinic light, while vinyl resins such as polyvinyl chloride and polyvinyl acetate spot and degrade. The rate of air oxidation of polyolefins such as polyethylene and polypropylene is materially accelerated by ultraviolet light.
Hindered 2,2,6,6-tetraalkyl-4-carboxylic acid ester piperidine compounds were described by Murayama et al in U.S. Pat. No. 3,640,928 as being light and heat stabilizers for synthetic polymers. These compounds have the general formula ##STR3## in which R.sub.1 and R.sub.2 are alkyl or form with the carbon atom to which they are attached a saturated alicyclic group or another piperidine ring, R.sub.3 is an acyl group, n is an integer of 1-3 and the salts thereof.
The Murayama compounds have proven to be particularly acceptable because they do not impart a discoloration of their own to the synthetic polymer. The compounds generally employed previously had been either highly colored, such as nickel compounds (normally green) and the 2-hydroxy-benzophenones (varying shades and intensities of yellow). They also showed very little tendency toward sublimation and extrudation, and had an excellent stabilizing action against both heat and light deterioration.
The Murayama et al patent has been followed by a large number of patent and literature disclosures relating to compounds including a 2,2,6,6-tetrasubstituted-4-piperidyl group attached to a base molecule of various structures. Reference can be made, for example, to the following U.S. Pat. Nos. 3,640,928, 3,920,659, 4,064,102, 4,102,858, 4,105,625, and 4,136,081.
Despite the effectiveness of the Murayama type compounds described in the literature the search has continued for new and more effective light stabilizers. It has been desired to realize an enhanced degree of light stabilization and also to obtain stabilizers which are even less likely to exude from the polymeric mass being stabilized.
Hillard et al U.S. Pat. Nos. 4,064,102, teaches esters of the formula ##STR4## in which R is hydrogen or alkyl, R' is hydrogen, hydroxy, or alkoxy, R" is alkyl, alkylene, cycloalkyl, cycloalkylene, arylene, aralkylene and alkynyl and in which n is an integer from 1 to 4. These esters are stated to provide superior stabilizing properties against photodegradation of synthetic polymers compared to the Murayama type compounds. See also Japan Kokai 77-91,875 and 77-139,071. It is desirable, however, to obtain compounds which have greater stabilizing properties and impair the physical properties to a lesser extent than the Hillard compounds.
It is accordingly the object of this invention to provide new and improved light stabilizers for organic materials which exhibit enhanced light stabilizing properties and/or resistance to exudation from the material being stabilized. This and other objects of the invention will become apparent to those skilled in the art from the following detailed disclosure.